Sensor assembly and a method of sensing

ABSTRACT

There is disclosed a sensor assembly and a method of sensing. The sensor assembly is for sensing a property associated with a structure of interest. The sensor assembly includes an elongate member constructed and arranged so as to be capable of assuming a structure-engaging form in which it is resiliently biased such that the member can engage with and grip the structure along at least part of the length of the member, and, at least one sensor supported by the elongate member.

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/375,424, filed Feb. 13, 2012, which is the National Phaseentry of PCT/GB2010/050897, filed May 28, 2010, which claims priority toGreat Britain Patent Application No. 0909525.8, filed Jun. 3, 2009. Thecontent of these applications is incorporated herein by reference intheir entirety.

The present invention relates to a sensor assembly and to a method ofsensing.

There are many applications where it is desirable to measure sameproperty associated with a structure, such as a beam or a pipe or agirder, etc. For example, it may be desired to measure the temperatureof a structure, or the pressure experienced by a structure or the stressor strain experienced by a structure. This may be done to ascertain theoperating conditions of the structure, or as a way of monitoring thestructure for signs of damage or signs indicative of imminent failure ofthe structure, etc.

To this end, an operator may visit the structure to perform aninspection of the structure with measuring equipment. However, this islabour intensive and time consuming, and prohibitive in many situations.Alternatively, sensors can be applied to the structure in a variety ofways.

For example, in the field of petrochemical pipelines it is known toinsert a Pipeline Inspection Gauge (PIG) into a pipe to sense propertiesof the pipe as the PIG advances through the pipe. This has thedisadvantage that the sensors are only temporarily in any particularposition in the pipe so that only a “snapshot” of the condition of thepipe can be obtained. Also, the type of sensing that can be performed bya PIG is constrained due to the fact that the PIG must move in the pipe.This can introduce acoustic noise and makes using certain types ofsensor difficult or impossible where the sensors need to have a reliablecontact with the structure, e.g. a strain gauge.

It is also known to attach suitable stationary sensors to a structure orto otherwise incorporate sensors in or on a structure to sense aproperty of interest so that a measurement can be obtained.

Discrete sensors can be attached to the structure, for example bybolting or gluing the sensor to the structure. These tend to have thedisadvantage that they only measure localised properties of thestructure, such as the temperature at a particular point where thesensor is located. To build a temperature profile along a length of thestructure, it would normally be necessary to apply many sensors to thestructure, which requires greater time and effort from an operator infitting the sensors.

An example of a sensor that can be attached to a structure is a straingauge, the use of which is well known per se to measure deformation inunderlying structures. Typically, the strain gauge consists of aninsulating flexible backing which supports a metallic foil pattern. Thegauge is attached to the structure by a suitable adhesive, such ascyanoacrylate. As the structure is deformed, the foil is correspondinglydeformed, causing its electrical resistance to change, which can bemeasured to give an indication of the deformation of the structure.Strain gauges however can be difficult to attach, for example where thestructure to be measured is hard to access by an operator. Strain gaugesusually need to be permanently attached, making it difficult orimpossible to replace, move, or remove a sensor once in place. Straingauges also usually measure relatively localised properties of thestructure and in most cases are limited to measuring strain (i.e.deformation) of the structure.

Alternatively, sensors may be incorporated into the structure at thetime of manufacture. This has the advantage that operator time is notrequired in fitting the sensors. However, it has the disadvantage thatthe manufacturing process is made more complicated and possibly theperformance of the structure is degraded due to having to accommodatethe sensors. Another disadvantage of this scheme is that sensorsincorporated into the structure at the time of manufacture usuallycannot be removed, meaning that it is generally impossible to repair orreplace a malfunctioning sensor, or upgrade a sensor, or even move asensor to a different part of the structure or to a different structure.

According to a first aspect of the present invention, there is provideda sensor assembly for sensing a property associated with a structure ofinterest, the assembly comprising:

-   -   an elongate member constructed and arranged so as to be capable        of assuming a structure-engaging form in which it is resiliently        biased such that the member can engage with and grip a said        structure along at least part of the length of the member; and,    -   at least one sensor supported by the elongate member.

The member is in effect capable of “clipping” to the structure, i.e.engaging with and gripping the structure due to its resiliency. Thismeans that the sensor assembly can be fitted to the structure withoutthe use of adhesives, bolts, etc. The clipping arrangement means thatthe member is simple to fit to a structure. The sensor assembly can beengaged to the outside of a structure, for example to the outside of abeam, or pipe, or girder, etc. Alternatively, the sensor assembly can beengaged to the “inside” of a structure where the structure is hollow orhas a concave cavity, such as a hollow pipe.

As well as attaching the sensor assembly to the structure, the grippingarrangement allows the structure to conform closely to the structure,meaning that the sensing assembly experiences similar conditions as thestructure. For example, stress, strain and vibration may be transmittedto the sensing assembly with a high degree of fidelity meaning thatwhatever property of the structure is being measured can be derivedaccurately from the data produced from the sensor of the sensorassembly. The clipping arrangement also means that the sensor assemblycan minimise acoustic noise due to friction caused by movement of thesensing apparatus relative to the structure.

The member being elongate means that the structure can be sensed along alength of the structure due to the longitudinal extent of the memberbeing substantially greater than the transverse extent of the memberwhen in the structure-engaging form. This means that a single sensorassembly requiring a single fitting operation to the structure can beused to monitor a substantial length of the structure over period oftime. The particular longitudinal extent of the member is determined bythe desired application of the sensor assembly. Nonetheless, inpreferred embodiments the longitudinal extent of the member may at least5 times, or at least 10 times, or at least 50 times, or at least 100times the transverse extent of the member when in the structure engagingform.

Thus the present invention provides a convenient way of monitoring alength of structure, where the sensor assembly can be retrofitted to thestructure, or fitted to the structure at the time of installation of thestructure, or even incorporated to the structure at the time ofmanufacture, as desired. The clipping arrangement also allows removal ofthe sensor assembly so that the sensor assembly can be repaired,upgraded, or moved in location, or retrieved for data recovery.

In principle, any suitable type of sensor can be used, such as forexample fibre optic sensor systems that are known to be embedded inflexible pipes as described below. Other types of sensors, such asstrain gauges or any other suitable sensor may be used. The sensor orsensors can extend the full length of the sensor assembly or at least asubstantial portion thereof. The sensor or sensors can be runcontinuously longitudinally along the member or a portion of the member(such as a continuous length of fibre optic sensor). Alternatively, aplurality of sensors may be positioned at various longitudinal positionson the member. In any event, it is preferred that the sensor or sensorsare arranged to be able to sense the structure at a plurality oflongitudinal positions.

Preferably the member is formed from a sheet-like material having firstand second longitudinal edges that is folded in on itself longitudinallyto form a tube or a longitudinal section of a tube when the member is inthe structure-engaging form. This provides a convenient form for thesensor assembly to be able to engage with and grip the structure.

This allows many convenient materials and manufacturing techniques to beused, for example, Fibre Reinforced Plastics or bistable materials, asdiscussed in more detail below. Using a sheet-like material allows themember to have thin walls. This arrangement also lends itself toembodiments where the sensor assembly is coiled or otherwise compactedfor storage. This can help minimise any impact of the sensor on thefunction of the structure being monitored. For example, this allows thesensor assembly to have a low profile relative to the surface of thestructure, so that the sensing assembly interferes as little as possiblewith the performance of the structure and influences as little aspossible the properties of the structure that it is desired to measure.Preferably the sensors do not protrude significantly or at all from thesurface of the member so that the low profile of the sensor assembly ismaintained.

In a preferred embodiment, in transverse cross section thestructure-engaging form of the member subtends an angle of at least 180degrees. Having an angle of at least 180 degrees allows the member toclip securely to the structure. In another preferred embodiment, intransverse cross section the structure-engaging form of the membersubtends an angle of about 360 degrees. In other words, the member formsan approximate tube. This can help provide an even more secureengagement between the sensor assembly and the structure as the membercan grip all around the structure. In another preferred embodiment, intransverse cross section the structure-engaging form of the membersubtends an angle of more than 360 degrees. In other words, there issome overlap between the longitudinal edges of the member in thisembodiment. Again, this can help provide a more secure engagement to thestructure.

In a preferred embodiment, in transverse cross section thestructure-engaging form of the member is generally curved. This providesa sensing apparatus that is highly versatile in being engageable to avariety of different structures. Preferred embodiments may be circularor oval. However, as described in more detail below, other crosssectional forms are possible. For example, the cross section can havestraight portions whilst being generally curved. In principle, the crosssection can even be polygonal. The precise shape of cross section chosenfor the member is in practice likely to depend on the application and inparticular the structure being monitored.

Preferably, the member is constructed and arranged so that it can beprogressively flattened and wound about an axis extending transverselyto the longitudinal extent of the member to form a coil so as to bereversibly configurable between a coiled form and an extended form,wherein the extended form is the structure-engaging form. The coiledform allows the sensor assembly to be conveniently stored when notdeployed. This can make storing and transporting the sensor assembly tothe site of the structure more convenient as well as making fitting thesensor assembly more convenient.

Preferably, both longitudinal opposed ends of the member are open endedso that the member can engage with and grip a said structure along thefull length of the member.

In embodiments, the member is constructed and arranged to allow thesensor assembly to follow bending of the structure of interest whilstmaintaining its grip on the structure and staying free fromdeformations, e.g. without buckling of the edges of the member. Theprecise amount of bending of the structure that the sensor assemblyshould be able to follow depends on the application of the sensorassembly and on the structure of interest and in particular on itsdiameter. Nonetheless, preferred embodiments of the sensor assembly areconstructed and arranged such that the peak tensile and compressivestrains of the member on the convex and concave sides of the bendrespectively may be of a minimum of 4%.

In an embodiment, the member is formed from a bistable material having afirst stable form in which it is coiled and generally flat in crosssection, and a second stable form in which it is extended and has thestructure-engaging form. Using a bistable material for the member meansthat the sensing assembly can be stable both when coiled and whenextended and when partially coiled and partially extended. This allowssafer and simpler storage of the sensing assembly without impacting itsability to engage with and grip the structure.

Preferably, the member is constructed and arranged to be reversiblyclippable to a said structure of interest. This allows the sensorassembly to be removed from the structure for upgrading, or to be movedto a different position on the structure or to a different structure.

Preferably, the at least one sensor includes a fibre optic sensor thatextends along at least a portion of the member.

Preferably, the member is formed from a laminate of at least two layersand said sensor is positioned between two of the layers. This protectsthe sensor from environmental damage.

Preferably, the sensor assembly comprises electronic apparatus incommunication with the sensor arranged to provide at least one of: i) adata logging system to allow readings from the sensor to be loggedsystem and ii) a data transmitting system to allow readings from thesensor to be transmitted to remote receiving apparatus.

According to a second aspect of the present invention there is providedin combination, deployment apparatus and a sensor assembly as describedabove, the deployment apparatus comprising:

-   -   a gripper for gripping a said structure of interest;    -   a holder for holding the sensor assembly when coiled;    -   actuating means for in use advancing the deployment apparatus        relative to the structure; and,    -   a guide member for progressively guiding the sensor assembly        from being wound to being clipped to the structure as the        deployment apparatus advances.

The combination may comprise a clamp dispenser arranged to automaticallyapply a clamp to the member to attach to attach the member to thestructure at one or more points along the length of the member to keepthe sensor assembly in position longitudinally on the structure.

According to a third aspect of the present invention there is provided amethod of sensing a property associated with a structure of interestusing a sensor assembly, the method comprising:

-   -   engaging an elongate member of the sensor assembly with the        structure of interest, the elongate member being resiliently        biased such that the member engages with and grips the structure        along at least part of the length of the member; and,    -   sensing a property associated with the structure with at least        one sensor supported by the elongate member.

Preferably, engaging the member with the structure comprisesprogressively engaging the member with the structure along the length ofthe member.

In an embodiment, the member is formed from a sheet-like material havingfirst and second longitudinal edges and when engaged with the structurethe member forms a tube or a longitudinal section of a tube, whereinengaging the member with the structure comprises: separating thelongitudinal edges of the member; moving the sensor assembly intoposition next to the structure; and, allowing the resiliency of themember to cause the member to engage with and grip the outside surfaceof the structure.

In another embodiment, engaging the member with the structure comprises:compressing the member; introducing the compressed member into a hollowportion of the structure; and, allowing the resiliency of the member tocause the member to engage with and grip the inside surface of thestructure.

In an embodiment, the method comprises securing the sensor assembly tothe structure at one or more points along the length of the member tokeep the sensor assembly in position longitudinally on the structure.This helps avoid slippage of the sensor assembly along the structure.

Preferably, the step of sensing with the sensor comprises at least oneof: a) sensing temperature; b) sensing pressure; c) sensing vibration;d) sensing stress; and e) sensing strain of the structure.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 shows an example of a sensor assembly according to an embodimentof the present invention;

FIG. 2 shows a partial cut away of the example of FIG. 1;

FIGS. 3 to 6 show in cross section further examples of sensor assembliesaccording to embodiments of the present invention;

FIG. 7 shows an example of a sensor assembly according to an embodimentof the present invention when engaged with a structure;

FIG. 8 shows an example of a structure for which the present inventionhas particular applicability;

FIG. 9 shows an example of deployment apparatus according to anembodiment of the present invention and FIGS. 9A and 9B show detailviews of elements of FIG. 9.

Referring to FIG. 1, an example is shown of a sensor assembly 1according to an embodiment of the present invention. The apparatus 1comprises an elongate member 2. The member is formed of a strip ofsheet-like material, i.e. the member is thin in cross section. Thethinness of the material will in practice depend on the application ofthe sensor assembly 1. Nonetheless, in preferred embodiments the membermay be less than 5 mm, or less than 2 mm, or less than 1 mm thin incross section. It is anticipated that in most cases the thinness of themember compared to the width of the member with deployed may be lessthan 40. In some examples (not shown) additional layers may attached tothe member 2 for various other purposes, such as floatation layers orlayer providing protection from impact damage. These addition layers maygive rise to higher overall thickness in the sensor assembly 1.

A plurality of sensors 3 are provided running along the length of theelongate member 2. In the present example, the sensors 3 are fibre opticsensors as described in more detail in the following description.

The member 2 as depicted has an extended portion 4 and a coiled portion5. The member 2 is capable of being progressively transformed betweenbeing completely coiled and being completely extended. The length of themember 2 (i.e. its longitudinal extent) when fully extended issubstantially greater than its width (i.e. its transverse extent) whenextended. The preferred length will in practice depend on theapplication chosen for the sensor assembly 1. Nonetheless, in preferredembodiments the length of the member 2 may at least 5 times, or at least10 times, or at least 50 times, or at least 100 times the width of themember 2.

As shown in more detail in FIG. 2, the extended portion 4 is resilientlybiased to have a cross section that is curved in the form of a partialcircle. The partial circle subtends an angle of about 200 degrees. Inother words, from the centre of the circle 7, the member 2 sweeps out apartial circle over an angle 8 of about 200 degrees. It is preferredthat the subtended angle is greater than 180 degrees so that the openingto the split tube is narrower than the maximum internal diameter of thetube. This enables the member to “clip” to a structure.

FIGS. 3 to 6 show other examples of cross sections for the member 2. InFIG. 2, the member 2 has a circular cross section subtending an angle ofabout 360 degrees, i.e. the member forms a circular tube with alongitudinal split where the edges 6 of the member meet. FIG. 4 shows amember 2 where the angle subtended is greater than 360 degrees, i.e. themember approximately forms a circular tube with some degree of overlapof the edges 6.

As shown by FIGS. 5 and 6, it is not essential to have a circular crosssection. The cross section is however preferably generally curved.However, this does not preclude the cross section having straightportions whilst being generally curved. Ovals and other continuous,non-circular arcs subtending over 180 deg in total can also be produced.

FIG. 7 shows a sensor assembly 1 according to an embodiment of thepresent invention when engaged with a structure 10. The structure 10 inthis example is a pipe having an approximately circular cross section.The sensor assembly 1 shown in FIG. 7 corresponds most closely to thesensor assembly 1 shown in FIG. 3, i.e. where the cross section of themember 2 is circular and subtends an angle of about 360 degrees. As willbe appreciated, it is preferred that the sensor assembly 1 chosen for aparticular application has a member cross section that matches therelevant surface of the structure to which it is being engaged topromote a good engagement and grip between the two. Thus, in the presentcase, a member having a circular cross section is most preferred for apipe 10 having a circular cross section.

The extended member 2 is resiliently biased to maintain its circularcross section. This allows the member 2 to engage with and grip theoutside surface of the structure 10. Because of the similarity of crosssectional shape of the member 2 and the shape of the structure 10, thesensor assembly 1 closely follows the structure 10. This enables thesensors 3 of the sensor assembly 1 to sense the properties of thestructure 10. For example, temperature, pressure and vibration of thepipe 10 will be coupled to the sensor assembly 1, allowing the sensor 3to sense these properties associated with the pipe 10 and allowing, withsuitable calibration, a measurement to be taken from the sensor data. Inthis particular example, the member 2 has hoop stiffness large enough togrip the structure 10 and at the same time a longitudinaltensile/compressive modulus low enough to allow the sensor assembly 1 tofollow bending of the structure of interest. This allows the sensorassembly 1 to grip and closely follow the structure as the structurebends or otherwise deforms allowing stress and strain to be transmittedto the sensor assembly 1 and so measured.

The preferred sensor 3 is a fibre optic cable sensor. Use of fibre opticcable sensors are known per se, for example for monitoring flexiblepipes, as discussed in more detail in the following description inrelation to FIG. 8. Nonetheless, other types of sensor can be used, forexample a strain gauge. An electronics system 9 is provided and attachedto an end of the sensor assembly 1. The electronics system may include adata-logger, power supply and associated instrumentation 9 connected tothe sensors 3. The data-logger logs the outputs of the sensors 3.Alternatively or additionally, the system 9 can comprise a transmitterfor relaying the sensor data to a remote receiver for analysis.

For the member 2, any material of sufficient resilience to be reversiblycoiled and extended, and to maintain a closure force sufficient toprovide good coupling between sensor assembly 1 and the underlyingstructure 10 could, in principle, be used. For example, suitableelastomeric polymers and even, in cases where the in-use strain is low,spring metals, could be used for the member 2. In practice, metals arenot preferred as they would have moduli significantly higher than thatof the polymers.

The use of a laminar construction with the sensors 3 embedded betweenlayers of the lamina gives an advantageous combination of straintransfer, protection of the fibre from impact, abrasion etc. The layersof the laminar could be polymer, elastomer or even metallic and bondingof the sensor be achieved by adhesively laminating or by “processing in”in the case of a laminar structure that is a fibre reinforced composite(FRC).

In principle, there are many different methods that are suitable toconstruct the sensor assembly 1. A particularly preferred method is tolaminate the sensors 3 between layers of fibre reinforced polymer (“FRP”hereafter). FRPs are known per se and are not described in detailherein. However, in brief, FRPs are composite materials made of apolymer matrix reinforced with fibres. The fibres are usuallyfiberglass, carbon, or aramid, while the polymer is usually an epoxy,vinylester or polyester thermosetting plastic. The use of fibrousmaterials mechanically enhances the strength and elasticity of theplastics. The original plastic material without fibre reinforcement isknown as the matrix. The matrix is a tough but relatively weak plasticthat is reinforced by stronger stiffer reinforcing filaments or fibres.The extent that strength and elasticity are enhanced in a fibrereinforced plastic depends on the mechanical properties of both thefibre and the matrix, their volume relative to one another, and thefibre length and orientation within the matrix.

The sensors 3 can be laminated either by incorporating them at the timeof consolidation, or by post bonding two or more layers of FRP shellusing an adhesive to hold the shells together with the sensors 3 inbetween the layers of the laminar. The use of FRP allows the mechanicalcharacteristics of the shell to be manipulated by varying the weight anddirection of fibres in the various layers in such a manner as to producesomething that can be tailored to the needs of a specific application ofthe sensor assembly 1.

Thus, a FRP is a particularly preferred material for making the member 2as it allows fine tuning of axial/torsional/hoop stiffness to beachieved by, for example, changing the angles and fibre content of thelayers. Nonetheless, other materials are possible. For example, a metalribbon can be used and tuning of its properties can be achieved bypunching holes and slots into the metal ribbon.

In a preferred embodiment, the material used for the member is abistable material, whether made of FRP or otherwise. Such a bistablemember comprises an extendable, coilable member that can be reconfiguredfrom a coiled or retracted state to an extended state. The sensorassembly 1 shown in FIG. 1 can be made optionally from a bistablematerial such that it has a first stable state in the coiled form,wherein the cross section of the member is generally flat and a secondstable state in the extended form, wherein the cross section of themember is curved as previously described. Preferably, the bistablemember 2 is capable of reversible configuration between its coiled andextended forms a plurality of times. Suitable structures are disclosedin the following international patent applications, each of which isincorporated here by reference: WO-A-88/08620, WO-A-97/35706,WO-A-99/62811, and WO-A-99/62812. Such bistable structures are availablefrom RolaTube Technology Limited of Lymington, United Kingdom.

As described in the above-referenced patent applications, such abistable member generally comprises material that creates a bias towardsconfiguring the material in the extended form (e.g. having a circularcross-section in this example), as well as material that creates a biasopposite to the first bias (e.g. one that biases the member towards itsflattened, retracted or coiled form). The member can comprise aresilient substrate, made of metal for example, which is biased towardthe extended form (e.g. biased toward making the member have a circularcross-section), laminated with a plastic layer that tends to bias themember towards the retracted form (e.g. having a flattenedcross-section). Alternatively, the member can comprise a strip or sheetof a thermoplastic material having prestressing means attached theretoor embedded therein. One particular example is a thermoplastic striphaving prestressed fibres therein (such as fibres of glass, carbon, orpolymeric materials). The fibres can be located at different anglesrelative to each other in the plane of the coiled member, such ascomprising one set of fibres that are longitudinally extending and asecond set of fibres that are transversely extending. Suchfibres-reinforced composite members (e.g. a thermoplastic resin, such aspolyethylene or polypropylene, with fibres of another material, such asglass, carbon, or aramid, embedded therein) are preferred for use in thepresent invention.

Whatever material is used to form the member 2, and whatever crosssection is used for the member 2, what is important is that the member 2is capable of engaging with and gripping the structure 10. Potentiallymany types of material, construction and cross section are usable,dependent upon the application, i.e. structure being monitored and whatproperties are being monitored.

As a general rule, it is preferred that the axial modulus of the member2 (and thus the bending stiffness of the sensor assembly 1 whendeployed) should be as low as is consistent with providing a member 2that has sufficient strength, impact resistance, abrasion resistance,etc. to survive the environment in which it is to be used, and in anyevent very significantly lower than that of the underlying structure 10.

As another general rule, it is preferred that the hoop modulus, withreference to the hoop bending “plane” (i.e. the force exerted to clampthe device onto the underlying structure) should be high enough toensure that the device cannot move relative to the underlying structure10 and to prevent buckling out the “seam” line formed by the edges 6 ofthe member 2 under bending where this is the concave bending face.

In the case of acoustic monitoring of the structure 10, the requirementsmay be different again. Flexing of the structure 10 may be minimal, soit is not essential to have low bending stiffness of the extended member2. In any case, it is always desired to have sufficient hoop closure ofthe member 2 to couple it closely to the structure 10 and it needs to beresilient enough to be coiled.

FIG. 8 shows a preferred application of the present invention. It isknown to use flexible pipe to transport production fluids, such as oiland/or gas and/or water, from one location to another. FIG. 8 shows ariser assembly 200 suitable for transporting production fluid such asoil and/or gas and/or water from a sub-sea location 201 to a floatingfacility 202. The flexible flow line 205 comprises a flexible pipe,wholly or in part, resting on the sea floor 204 or buried below the seafloor and used in a static application. The floating facility may beprovided by a platform and/or buoy or, as in this example, a ship 202.The riser 200 is provided as a flexible riser, that is to say a flexiblepipe connecting the ship to the sea floor installation. Portions offlexible pipe body can be utilised as a flowline 205 or jumper 206. Thepipe body is generally built up as a structure including metallic andpolymer layers. The pipe structure allows large deflections withoutcausing bending stresses and strains that impair the functionality ofthe pipe over its lifetime. Flexible pipe is particularly useful inconnecting a sub-sea location to a sea level location.

It is desirable to be able to detect the mechanical impacts on theflexible pipe 200 along its length, which may comprise great andunpredictable forces or very varying temperature impacts. It isparticularly desirable to monitor flexible pipes 200 as they have acomplex construction and the consequences of failure are potentiallycatastrophic, for example lethal danger to crew and platform workers aswell as presenting an extreme environmental hazard. Also lost productiondue to failure of a pipe can be very costly and such pipes are typicallyvery difficult and expensive to replace. It is therefore highlydesirable to monitor the pipe 200 and detect pipe degradation beforefailure or danger to personnel. In addition, existing calculations forpipe life spans are very conservative with high safety factors. Byeffectively monitoring the pipe 200, the pipes can be utilised withgreater efficiency.

To this end, it is known in the art to monitor flexible pipes 200 withbuilt-in fibre-optical sensors or other sensors such as strain gauges inselected layers of the flexible pipe. For example, US-A-2005/0210961discloses equipping a flexible pipe with linear extensometers or gaugesdesigned to detect twists in the pipe. US-A-2004/0168521 disclosesproviding a groove in one of the layers of a flexible pipe into which afibre optic sensor is secured during manufacture of the flexible pipe,such that the fibre optic sensor is sandwiched within the body of theflexible pipe.

In this way various properties associated with the flexible pipes 200can be monitored, including strain (e.g. for possible metal fatigue),temperature (e.g. for possible overheating or detection of breaches),pressure (e.g. for detecting excess build up) and the ingress of gasinto the internal layers of the flexible pipe (e.g. which can lead tocorrosion). Sensing using fibre optic in this way is generally known inthe art and is therefore not described in detail herein.

The disadvantage of such arrangements is that the sensors must beincorporated into the pipes during manufacture. Pipes 200 manufacturedwithout sensors cannot of course be monitored in this way. Also, oncefitted, the sensors cannot be removed from the pipes, for example forupgrading or repair, nor moved to a different position on the pipe or toanother pipe.

For these reasons, the sensor assembly 1 according to an embodiment ofthe present invention is particularly preferred for use with a flexiblesub-sea pipe 200 of this sort. As described above, the sensor assembly 1can be clipped to a pipe and used to monitor the performance of the pipe200.

As will be appreciated, it is preferred to use automation where it isdesired to retrofit a sensor assembly 1 to a flexible pipe 200 that isalready in position, i.e. sub-sea. FIG. 9 shows an example of anautomated apparatus 100, i.e. a remotely operated vehicle (ROV), fordeploying the sensor assembly on a sub-sea flexible pipe 200.

The sensor assembly 1 is initially provided to the deployment apparatus100 in its coiled form. The electronics system 9 is pre-installed on theend of the coiled sensor assembly 1, for example in the space within thecentre of the coil. Making the connections between the sensors 3 and theinstrumentation before deployment avoids the need for connections to bemade sub-sea. The size of this space in the centre of the coil can bevaried in order to provide a suitable envelope for this equipment.

The deployment apparatus 100 is fitted with a dispenser 101 for holdingthe coiled sensor assembly 1 and guiding the sensor assembly 1 intoposition on the flexible pipe 200. The deployment apparatus 100 has amotor 102 and power source (not shown), such as a battery, for poweringthe deployment of the sensor assembly 1. The deployment apparatus 100has a calliper 103 at one end comprising two actuated, pivoted arms 103a, 103 b which are actuated to fit over the flexible pipe 200 andextended sensor assembly 1, as shown in FIG. 9A. The calliper 103retains the deployment apparatus 100 in position with respect to theflexible pipe 200 during the installation procedure and helps ensurethat the sensor assembly 1 is accurately positioned relative to theflexible pipe 200 as it is deployed. The calliper 103 also ensures atight, accurate fit between the sensor assembly 1 and the pipe 200.Optionally, a secondary guide (not shown) may be provided at theopposite end of the deployment apparatus 100, in order to maintainalignment and avoid stressing the dispenser mountings.

The coiled sensor assembly 1 is fitted behind the guide calliper 103 andhas a narrow, protruding tongue 11 at its free end that is left extendedthrough the calliper 103. This allows the end of the sensor assembly 1to be secured to the pipe 200 prior to deployment. Preferably, a bandclamp 12 is used for securing the tongue 11, as shown in FIG. 9B. Thedeployment apparatus 100 has a band clamp deployment system 104 forapplying the band clamp 12 such that the tongue 11 is secured to theflexible pipe 200. The band clamp deployment system 104 can beincorporated into the dispenser 101, as shown, or alternatively can beseparately deployed. Although the resiliency of the member 2 issufficient to grip the structure in most situations, optionally, thesensor assembly 1 can also be clamped to the flexible pipe 200 at morethan one location along its length to help keep the sensor assembly 1 inplace on the structure. It is preferred to have at least a clamp 12 atthe start of the run to keep the first end of the sensor assembly 1 inplace, especially during deployment, and a clamp 12 at the opposite endof the sensor assembly 1 to ensure absence of slippage along the lengthof the deployed sensor assembly 1. Intermediate clamps 12 may, or maynot, be desired depending on such factors as the length of run and thediameter of the pipe 200. Data defining the need for and spacing ofclamps can be obtained by carrying out preliminary testing.

The deployment process is as follows:—

-   1) The deployment apparatus 100 locks the guide calliper 103 over    the pipe 200.-   2) A band clamp 12 is deployed to lock the tongue 11 and the end of    the sensor assembly 1 onto the pipe 200.-   3) The motor moves the deployment apparatus 100 moves along the pipe    200, deploying the sensor assembly 1 along its length.

The motor 102 may move the deployment apparatus 100 along the flexiblepipe 200 and in so doing pull the sensor assembly 1 out of the dispenser101 as the deployment apparatus 100 advances. Alternatively, the motor102 may power the dispenser 101 itself, so that the sensor assembly 1 isprogressively unwound under the power of the motor 102, and using thisto push the deployment apparatus 100.

-   4) As the sensor assembly 1 is progressively unwound and clipped    along its length to the pipe 200, band clamps 12 are applied, as    required, along the length of the sensor assembly 1.-   5) The end of the sensor assembly 1 clears the dispenser 101 and is    optionally clamped in place, with the electronics system 9 attached    as shown for example in FIG. 7.-   6) The calliper 103 is disengaged and the deployment apparatus 100    can be recovered.

Suitable mechanisms for dispensing the sensor assembly 1 under power, asnoted in 3 above, are known per se in the prior art. Such mechanisms aremanufactured for example by RolaTube Technology Limited of Lymington,United Kingdom for the powered deployment of bistable coiled members.These currently find application in such areas as the monitoring ofnuclear power stations, and the deployment of military hardware. Suchmechanisms can be simple and rugged and no problems would be anticipatedin producing a mechanism suitable for this purpose. The final decisionon whether to power the dispenser 101 or to rely on the drive of thedeployment apparatus 100 for deployment can be made depending on what isdesirable for the particular implementation on a case by case basis.

Depending on the length of sensor assembly 1 to be deployed and the sizeof the pipe 200 to be monitored it may prove desirable to have adispenser 101 large enough to hold more than one sensor assembly 1monitoring element. Multiple installations could thus be made on singledives.

Embodiments of the present invention have been described with particularreference to the examples illustrated. However, it will be appreciatedthat variations and modifications may be made to the examples describedwithin the scope of the present invention.

1. A sensor assembly for sensing a property associated with a structureof interest, the assembly comprising: an elongate member constructed andarranged so as to be capable of assuming a structure-engaging form inwhich it is resiliently biased such that the member can engage with andgrip said structure along at least part of the length of the member,wherein the longitudinal extent of the member is at least 5 times thetransverse extent of the member when in the structure-engaging form;and, at least one fibre optic sensor fixed to the elongate member andextending longitudinally along the member, wherein the sensor assemblygrips and follows a length of the structure as the structure bends ordeforms and so allows the fibre optic sensor to sense a propertyassociated with that length of the structure.
 2. A sensor assemblyaccording to claim 1, wherein the member is formed from a sheet-likematerial having first and second longitudinal edges that is folded in onitself longitudinally to form a tube or a longitudinal section of a tubewhen the member is in the structure-engaging form.
 3. A sensor assemblyaccording to claim 1, wherein in transverse cross section thestructure-engaging form of the member subtends an angle of at least 180degrees.
 4. A sensor assembly according to claim 1, wherein intransverse cross section the structure-engaging form of the membersubtends an angle of about 360 degrees.
 5. A sensor assembly accordingto claim 1, wherein in transverse cross section the structure-engagingform of the member subtends an angle of more than 360 degrees.
 6. Asensor assembly according to claim 1, wherein in transverse crosssection the structure-engaging form of the member is generally curved.7. A sensor assembly according to claim 1, wherein the member isconstructed and arranged so that it can be progressively flattened andwound about an axis extending transversely to the longitudinal extent ofthe member to form a coil so as to be reversibly configurable between acoiled form and an extended form, wherein the extended form is thestructure-engaging form.
 8. A sensor assembly according to claim 1,wherein both longitudinal opposed ends of the member are open ended sothat the member can engage with and grip said structure along the fulllength of the member.
 9. A sensor assembly according to claim 1, whereinthe member is formed from a bistable material having a first stable formin which it is coiled and generally flat in cross section, and a secondstable form in which it is extended and has the structure-engaging form.10. A sensor assembly according to claim 1, wherein the member isconstructed and arranged to be reversibly attachable to said structure.11. A sensor assembly according to claim 1, wherein the member is formedfrom a laminate of at least two layers and said sensor is positionedbetween two of the layers.
 12. A sensor assembly according to claim 1,comprising electronic apparatus in communication with the sensorarranged to provide at least one of: i) a data logging system to allowreadings from the sensor to be logged system and ii) a data transmittingsystem to allow readings from the sensor to be transmitted to remotereceiving apparatus.
 13. A method of sensing a property associated witha structure of interest using a sensor assembly, the method comprising:engaging an elongate member of the sensor assembly with the structure ofinterest, the elongate member being resiliently biased such that themember engages with and grips the structure along at least part of thelength of the member, wherein the longitudinal extent of the member isat least 5 times the transverse extent of the member when in thestructure-engaging form; and, sensing a property associated with thestructure with at least one fibre optic sensor fixed to the elongatemember and extending longitudinally along the elongate member, whereinthe sensor assembly grips and follows a length of the structure as thestructure bends or deforms and so allows the fibre optic sensor to sensea property associated with that length of the structure.
 14. A methodaccording to claim 13, wherein engaging the member with the structurecomprises progressively engaging the member with the structure along thelength of the member.
 15. A method according to claim 13, wherein themember is constructed and arranged so that it can be progressivelyflattened and wound about an axis extending transversely to thelongitudinal extent of the member to form a coil so as to be reversiblyconfigurable between a coiled form and an extended form, the methodcomprising: progressively unwinding the sensor assembly from its coiledform and engaging the extended portion with the structure.
 16. A methodaccording to claim 15, wherein the member is formed from a bistablematerial having a first stable form when it is in the coiled form and asecond stable form when it is in the extended form.
 17. A methodaccording to claim 13, wherein the member is formed from a sheet-likematerial having first and second longitudinal edges and when engagedwith the structure the member forms a tube or a longitudinal section ofa tube, wherein engaging the member with the structure comprises:separating the longitudinal edges of the member; moving the sensorassembly into position next to the structure; and, allowing theresiliency of the member to cause the member to engage with and grip theoutside surface of the structure.
 18. A method according to claim 13,comprising securing the sensor assembly to the structure at one or morepoints along the length of the member to keep the sensor assembly inposition longitudinally on the structure.
 19. A method according toclaim 13, comprising removing the sensor assembly from the structure.20. A method according to claim 13, where the step of sensing with thesensor comprises at least one of: a) sensing temperature; b) sensingpressure; c) sensing vibration; d) sensing stress; and e) sensing strainof the structure.